Powdery Mildew
Disease · Commonly affects: Squash and zucchini, Cucumbers, Pumpkins, Melons, Peas, Grapes
Powdery mildew is a common fungal disease caused by several closely related fungi that coat leaf surfaces with a white, dusty growth. Home gardeners notice it because it spreads quickly in late summer and can weaken plants and cut into yields, especially on squash and other favorites.
How to identify it
Look for white to grayish powdery patches, usually first on the upper sides of older leaves and on stems. The spots begin as small circles that expand and merge until they cover the leaf, and the growth can be rubbed off with a finger. Affected leaves often turn yellow, then brown and dry, and new growth may be stunted or distorted.
Damage
The fungal film blocks sunlight and draws nutrients from the leaf, which reduces photosynthesis and saps the plant's vigor. Heavily infected leaves yellow and drop early, so plants produce fewer and smaller fruits, and exposed fruit can become sunscalded or ripen poorly.
Prevention
Choose resistant or tolerant varieties when they are available, since many cucurbit and pea cultivars are bred to shrug off this disease. Space and stake plants for good airflow, plant in full sun, and avoid heavy nitrogen feeding that pushes soft, susceptible growth. At season's end, clear away and dispose of infected debris rather than composting it, and rotate cucurbits and other prone crops to a new bed the following year.
How to manage it
Pinch or prune off the first infected leaves and shoots as soon as you spot them, and put that tissue in the trash to slow the spread. Thin crowded stems to open the canopy so leaves dry faster, and water at the base of the plant in the morning to keep foliage and humidity down around the leaves. If a plant is heavily coated late in the season, pull it to protect nearby healthy crops.
General guidance for US home gardens, focused on prevention and non-chemical control. For a confirmed diagnosis and options specific to your area, contact your local Cooperative Extension office. Page updated Aug 4, 2026.